Conventional liquid roller coating lines using coil-to-coil transfer of metal strip from the unwind reels to the rewind reels, apply solvent or water-based paints, lacquers and coatings to the strip through the use of reverse or forward roller coating machines or similar application devices. The "wet" strip is then passed through catenary, flotation or combination catenary/flotation ovens for driving through a cross-linking process. Environmental regulations, however, which have become more stringent in the past two decades, now require the containment and disposal of such solvents. Thus, costly solvent containment and incineration systems for eliminating the volatile organic compounds in the paint have to be utilized in conjunction with these coating processes. Moreover, there is a finite limit to the thickness of coating that can be applied using this method. Further, appearance problems often arise when the paint is rolled onto embossed surfaces.
Due to the aforementioned drawbacks of liquid roller coating lines, the technique of powder coating strip metal has been developed in the industry. Utilization of this technique normally involves applying electrostatically charged dry plastic powder to the strip, and then passing this strip with plastic powder through an oven where the powder is melted and cured through a cross-linking process.
These plastic powders have the advantage of including only a fraction of volatile solvents associated with liquid paints, and thus are environmentally friendly and do not require expensive incineration and recovery systems.
A number of devices for applying a powder coating to a coil of strip have been devised. Currently, devices exist which can transport the strip in coil form through the powder coating application apparatus at speeds of approximately 200 feet per minute, the maximum speed limited by the powder application rate of the powder coating application apparatus. Continuously powder-coated strip, though usable for many applications, has several drawbacks. In particular, problems arise when "blanks" are made from pre-coated coils. "Blanks" are sections of strip which have been processed, for example, by having edges sheared and holes punched in it. The punching of holes, and also the shearing of these blanks (thus creating "sheared blanks") from the coil leaves exposed sheared ends, edges and raw edges at the punched holes where special rust prevention is often particularly necessary when the blank is used in the design and formation of commercial products. Special rust protection is especially critical if the finished product is subjected to high humidity, such as in coastal regions, to prevent "filiform" rust and ensure the integrity of the coating. Other examples of powder coating continuous elongated articles (such as wires) prior to punching and forming are shown in U.S. Pat. Nos. 3,439,649, 3,560,239, 3,396,699, 4,244,985 and 4,325,982.
Processes and apparatusses for forming a plastic coating on a metal strip are known. These processes normally include the steps of applying electrostatically charged powder to the strip within an enclosed chamber, heating the strip rapidly with an induction heater to melt the powder, and then maintaining the temperature of the coated strip to above the melting point of the plastic powder for a certain period of time before quenching. These coating lines can optionally include a press which pre-punches a strip into a continuous series of blanks, the pre-punched strip being passed through the powder coating and heating steps before being cut into lengths by a shear. Because the shearing of the blanks occurs after coating, however, the ends of each blank are left as unprotected, exposed raw metal. Detrimentally, these exposed sheared ends are likely to be subjected to corrosive elements when the blank is formed into a part and utilized in a commercial product. This is due to the fact that the ends of the blank are typically bent around to form a rounded edge during part formation, such that the raw end surfaces are typically proximate the exterior of the product. Furthermore, corrosion in these areas will become visible sooner than any untreated surfaces in the interior of the part.
Complete powder coating of already formed parts is known, however, the line speed must be extremely slow to ensure that the parts, which must be hung from a moving conveyor, are completely covered. An improved, faster technique for the powder coating of pre-sheared and punched blanks has been incorporated in some processing lines outside the United States. These machines have the advantage that the entire pre-sheared and punched blank may be chemically treated and powder coated one side at a time, including the sheared ends and punched holes. These coated blanks are then formed into parts after the powder coating operation. This type of chemical pre-treatment and powder coating of the entire blank provides rust protection throughout the part, including its edges. Such protection is, as in certain household appliances, such as refrigerators and washing machines, mandatory to prevent "filiform" rust. The coating speed of these machines is disadvantageously limited, however, by the speed at which the conveyed pre-sheared blanks may be unstacked and restacked. Currently, the maximum line speed attained is approximately 50 feet per minute, especially when long blanks, such as those used for refrigerator wrappers or the like, are being used.